?How many extraterrestrial civilizations are there in the Milky Way
This artist's impression depicts Kepler-186f, the first confirmed Earth-sized planet, orbiting a distant star in the habitable zone.
photograph by illustration by nasa ames/jpl-caltech/t. pyle
The Milky Way is home to a multitude of habitable planets, and about half of the solar-type stars have Earth-sized worlds as neighbors that could harbor life.
This should delight hunters of extraterrestrial life: more than 300 million worlds with conditions suitable for life as we know it are scattered in the Milky Way. According to a new analysis , about half of these solar-like stars harbor rocky planets in the habitable zone, where water in a liquid state could pool or drain on the surface.
"It's a scientific discovery that we've all been waiting for," says Natalie Batalha, an astronomer at the University of California, Santa Cruz, who took part in the new study.
Published in the Astronomical Journal , this discovery identifies a crucial number in the Drake equation. Developed in 1961 by my father, Frank Drake, this equation establishes a framework for estimating the number of detectable civilizations in the Milky Way. And we now know the first variables of the formula, namely the rate of formation of solar-type stars, the proportion of these stars that have planets and the number of habitable worlds per stellar system.
The probability of solar-like stars having Earth-like planets "could be one in a thousand or one in a million, nobody really knows," says SETI Institute astronomer Seth Shostak . (Search for Extraterrestrial Intelligence) who did not take part in the new study.
Astronomers used data collected by Kepler, NASA's planet-detecting space telescope, to estimate the number of such planets. For nine years, Kepler stargazed and watched for brief flickers caused by orbiting planets that hid some of their starlight. In 2018, as its mission drew to a close, Kepler had detected around 2,800 exoplanets , most of which had nothing to do with those orbiting our Sun.
But the space telescope had one primary purpose: to determine how widespread Earth-like planets were. The help of the European Space Agency's Gaia satellite , which observes the stars of the galaxy, was needed to make this estimate. Thanks to the observations of Gaia, scientists were finally able to discover that the Milky Way was home to hundreds of millions of Earth-sized planets orbiting around solar-type stars and that the closest was most certainly within 20 light years of our solar system.
THE DRAKE EQUATION
The Drake equation uses seven variables to estimate the number of detectable civilizations in the Milky Way. It takes into account factors such as the share of solar-like stars with planetary systems and the number of habitable planets in each of these systems. From there, the equation takes into consideration how often life evolves on worlds under ideal conditions and how often these life forms eventually develop detectable technologies. In its original form, the equation assumes that extraterrestrials with technological know-how would evolve on the planets orbiting around solar-like stars.
“When astronomers talk about finding these planets, everyone talks about the Drake equation,” says Jason Wright , an astronomer at Pennsylvania State University who studies potentially habitable planets but has no idea. did not take part in the study. "Each of us has that in mind when we make that calculation."
It took more than half a century for scientists to begin to determine the number of planets that could support life. In 1961, astronomers believed that the Sun was the only star with orbiting planets, and although planet formation theories suggested that exoplanets could be numerous, there was no empirical evidence that they existed. But, over the past decade, it has become clear that these planets are extremely widespread and outnumber stars in the Milky Way. On average, almost one planet (at least) orbits around each star.
This discovery was “a big step forward,” says Jason Wright. "This meant that many sites suitable for life as we know it had potentially been detected." However, the next factor in the Drake equation, namely the number of habitable planets per planetary system, was more difficult to calculate, says Natalie Batalha.
POTENTIALLY HABITABLE PLANETS
It is by observing the drops in light caused by the passage of the planets in front of the stars and briefly obscuring part of the light of these that Kepler distinguishes the distant worlds. Based on the amount of starlight occulted and the frequency of the phenomenon, scientists can determine the size of the planet and the time it takes to orbit its star. Using this approach, Kepler has detected thousands of exoplanets of all sizes and all types of orbits. But the scientists were above all in search of a datum: to know what was the share of planets similar to the Earth, that is to say rocky, with a temperate climate and gravitating around stars of the solar type.
Early estimates suggested that perhaps 20% of solar-type stars harbor a planet that meets these criteria. We know today that this figure is closer to 50%, if not more.
"It's better than I thought. I always told the public that one out of four or five stars had planets. This result is a pleasant surprise”, explains Natalie Batalha. "Each solar-type star is guaranteed, on average, to have at least one potentially habitable planet."
However, calculating the frequency of these planets posed unforeseen challenges. The stars observed by Kepler were far more active than scientists expected and produced signals that could mimic or blur the signatures of transiting planets. In addition, the space telescope was capricious and required periodic maneuvers that complicated observations.
To reach their conclusion, Natalie Batalha and her colleagues combined data collected by Kepler and Gaia , which tracks and describes a billion nearby stars. Thanks to Kepler, the researchers identified planets with a radius between 0.5 and 1.5 times that of Earth and which would probably be more rocky than gaseous. As for the Gaia satellite, it allowed them to obtain the temperatures and size of the stars around which these planets orbit.
Rather than basing a planet's potential habitability on how far it is from a star, the team calculated how much energy hits each other. From there, she was able to select the planets where the temperatures would favor the accumulation of liquid water on the surface.
Once the team had a sample size of sun-like stars that included rocky, temperate planets, they were able to estimate how many such planets existed in the entire galaxy. According to her, between 37 and 60% of the solar-type stars of the Milky Way would harbor a temperate Earth-sized planet. However, by performing a more forward-thinking calculation of the energy required for a planet to be temperate, scientists found that 58-88% of solar-type stars would have such a planet.
Of course, the fact that a planet is in the habitable zone actually suitable for life is determined by many factors. Characteristics of the planets, such as magnetic fields, atmospheres, water content and plate tectonics play a role, but remain difficult to observe on small, distant worlds.
However, “this study really digs into the question of the number of sites harboring life,” says Jason Wright. “The authors calculated the most probable distance to the nearest planet meeting these characteristics and they found it in our celestial garden”. This planet would be located at a maximum of 20 light years, while four others would evolve at a maximum of 33 light years.
HABITABILITY AND CIVILIZATION
Now that astronomers have a good grasp of the question of how many Earth-like planets are scattered across the galaxy, they can continue their examination of the variables in the Drake equation. Many of the remaining factors will be difficult to determine, including the crucial questions of how often extraterrestrials develop technologies we might detect and how long such civilizations are detectable.
Another open question is whether or not scientists should include non-solar-like stars, given that several Earth-sized planets have been discovered around smaller, cooler stars. Also, worlds other than planets might also need to be considered, because even though most of those detected by Kepler are large and gaseous, "they could harbor forest moons like Endor" in Star Wars . , says Jason Wright. “Or I guess like Pandora in Avatar ”.
Astronomers are on the verge of determining the next factor in the equation, namely the share of habitable planets on which life evolves. As the exploration of our solar system continues, we find that the list of habitable worlds is long and diverse. For example, Mars or Europa, the icy moon of Jupiter, could harbor the presence of microbiological life. Even the toxic clouds floating above Venus could potentially harbor life forms.
"If it's happened more than once in the solar system, you'll get that estimate pretty quickly," says Jason Wright.
Finding even a single example of the presence of life outside Earth would prove that biology is not the result of some extraordinary cosmic fluke, but rather a likely result if the necessary ingredients are in place. For many astronomers, given the number of habitable planets in the universe, the presence of life is almost inevitable.
However, as my father so aptly puts it, the final variables in the Drake equation, the ones that will tell us whether Earth alone is home to tech-savvy organisms in the galaxy, will remain a mystery until we hear the whispers extraterrestrial worlds.
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